In a semiconductor device such as a semiconductor power module oriented toward railroads, a base plate having heat dissipation and a case formed of a resin or the like containing a semiconductor chip are fixed to each other mainly with a metal tapping screw in terms of assembling property, fixing strength, and the like.
A screw-hole is formed to have a depth longer than that of the screw to prevent the bottom surface of the screw-hole in the case from interfering with the screw. For this reason, there is a cavity between the screw end and the bottom surface of the screw-hole when the screw is fastened. This cavity also has a role of absorbing swarf of the case made when the tapping screw screws in the screw-hole.
A semiconductor module oriented toward railroads normally treats a large amount of power. Thus, when an electric power is applied across high-potential parts (collector and emitter of a semiconductor chip, electrodes and metal wires coupled thereto, and the like) and low-potential parts, partial discharge may occur at an electric-field-concentrated part in an insulator.
The partial discharge serves as an index of insulating deterioration. For this reason, in a semiconductor power module oriented toward railroads, conditions related to partial discharge withstand voltage must be satisfied in application of a predetermined voltage (for example, when a rated voltage is 6.5 kV, 5.1 kV).
However, an electric field between the distal end of the metal tapping screw serving as the low-potential part and the high-potential part (for example, an electrode or a metal wire) having the minimum spatial distance from the distal end becomes strong to cause partial discharge in a cavity between the screw end and the bottom surface of the screw-hole so as to disadvantageously fall short of a partial discharge withstand voltage.
In recent years, in a next-generation power modules each using a wide-gap semiconductor chip such as an SiC or GaN the development and production of which have advanced, an increase in withstand voltage of the module has been advanced (for example, a related voltage 13 kV or 16 kV). An increase in withstand voltage of the module makes a request for the partial discharge withstand voltage more strict. Thus, the improvement in partial discharge withstand voltage is a technical issue essential for a higher withstand voltage of the next-generation power module.
In order to solve the issue, a semiconductor device in which a filler made of a high-withstand-voltage resin is filled in a cavity between the distal end of a metal tapping screw and the bottom surface of a screw-hole in a resin case and heated and hardened is proposed (for example, see Patent Document 1). The cavity is filled with the filler as described above to improve a withstand voltage. A semiconductor device including a filler hardened in advance and a method of manufacturing the semiconductor device are also proposed (for example, see Patent Document 2). The screw is fastened by hardening the filler in advance to make it possible to suppress a fluctuation in partial discharge characteristics.